1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Related Background Art
Up to now, as an image forming apparatus for forming an image on a recording material such as paper, there has been known an image forming apparatus which uses an electrophotographic process.
For example, a color image forming apparatus primarily transfers plural colors of toners, which are formed on plural photosensitive drums, onto an intermediate transfer member sequentially to form toner images. After that, the apparatus secondarily transfers the toner images on the intermediate transfer member onto a recording material, and heat-fixes the toner images on the recording material by a fixing device to thereby form a color image.
In addition, in the image forming apparatus using the electrophotographic process, in order to cope with various recording materials (media), for example, in the case in which an image is formed on cardboard, rough paper, OHT, or the like, it is desirable to perform a fixing step at a process speed lower than a normal image forming speed in order to secure a fixing property and image transparency.
Further, in the case in which an image is formed on a thick paper (e.g., cardboard), rough paper, OHT, or the like, it is conceivable that a method of performing an image forming operation at a second process speed lower than a normal process speed (processing speed of an image forming operation such as a conveying speed of a recording material) is adopted. However, all steps of an image forming process (charging, exposure, development, primary transfer, secondary transfer, and fixing) of the electrophotographic process may be performed at a process speed associated with each medium. However, it is necessary to perform optimization of an applied bias voltage in charging, development, transfer, and the like with respect to each process speed in order to cope with plural process speeds. In particular, it is very difficult to optimally set a charging bias voltage and a development bias voltage with respect to each process speed such that an image of a desired quality is formed with respect to a structure and a composition of a photosensitive member and a composition of a toner.
Thus, it is conceivable that a method of performing an image forming process up to transfer of toner images onto a recording material at a first process speed and performing a fixing step for heat-fixing the toner images on the recording material at a second process speed is adopted.
With the former method, if a sufficient conveying distance (at least a length of the recording material) is secured between a secondary transfer unit and a fixing unit, it is possible to perform only the fixing step at the second process speed. However, it is difficult to reduce a size of the apparatus because the sufficient conveying distance is required between the secondary transfer unit and the fixing unit.
Thus, it is possible to always stabilize an image quality at the time of image formation regardless of a set mode by performing the image forming process from the charging step to the primary transfer step while keeping a constant process speed (first process speed) and switching a process speed in the middle of the image forming process in the secondary transfer step and the fixing step according to a mode (setting on whether the fixing step is performed at the first process speed or the second process speed).
However, in the case in which a structure for performing the charging step to the primary transfer step at the first process speed and performing the secondary transfer step and the fixing step at the second process step is adopted, it is necessary to adjust a positional relation between an image leading end and a recording material leading end (distance from the recording material leading end to the image leading end) according to fluctuations in the number of rotations of a motor at the time when the process speed is changed, time lag at the time of building-up and building-down of rotation of the motor, and the like.